1. Field of the Invention
This invention relates to a method for writing data into a data memory, such as a screen memory which is used in a CRT or plasma display device or a printer and in which data per pixel unit is stored, and particularly to a method for writing data into a memory capable of writing data with a word as a unit and of writing data at a high speed even if a so-called shift write operation, in which a pattern data is written extending between the two addresses, is required, thereby allowing to relax the burden on an associated software program and to increase system efficiency.
2. Description of the Prior Art
In data processing devices, such as office computers, word processors and personal computers, use is typically made of a CRT or plasma display device and, as an output device, various printers, such as wire-dot type impact printers, ink-jet printers and electrophotographic printers, for example, employing a laser beam, are used. In such display devices and printers, there is typically provided a data memory for processing input/output data.
FIG. 1 shows in block form a system structure of a typical prior art office computer. As shown, the system includes a central processing unit (CPU) 1, an internal memory 2, an external memory 3, a keyboard 4, a CRT display device 5, a printer 6, a character pattern memory 7 and a system bus 8. In a data processing system such as an office computer, the CRT display device 5, keyboard 4, printer 6 and the external memory 3 like a floppy disc are connected to the CPU 1 via the system bus 8. Thus, when an operator desires to process the data stored in the external memory 3 or the data input through the keyboard 4, he or she carries out required operations by looking at the information displayed on the screen of CRT display device 5. Then, in accordance with various instructions fed through the keyboard 4, the CPU 1 controls the operations of various components.
In such a case, if it is desired to enhance the quality of graphic patterns and characters to be printed or displayed, then a memory having an unacceptably large capacity as compared with the prior art character code type processing system is required because data must be processed pixel by pixel. A read/write operation of data for such large capacity data memory is normally carried out under the control of a software program. However, such a software control requires a relatively long time period so that there is brought about a disadvantage of lowering the writing speed. Furthermore, in order to enhance the quality of displayed or printed information, it is necessary to arrange characters spaced apart at an appropriate spacing along a character line, and, thus, shift processing of data is also called for. If such shift processing has been implemented, the data to be processed by a word as a unit are almost always written extending between the two adjacent addresses. In such a case, the burden on an associated software program is also increased thereby causing a further reduction in processing speed.
Under the circumstances, in order to increase the data write-in speed even with the above-described shift processing operation, there is proposed a method for sharing the functions between software and hardware as described in a patent application, entitled "BIT IMAGE MEMORY PROCESSING SYSTEM", and filed in the Japanese Patent Office on May 6, 1982. According to the proposed method, the mask register and shift register is formed by hardware and its control is shared by software, whereby shift data is mask-processed and written into a memory and the similar processing is carried out also for shifted out data, so that data may be written into two addresses in two cycles in total. However, in such a method requiring two cycles, more software processing time is required correspondingly.
There is also proposed another method in which a shift operation of data is carried out by hardware and a control over mask operation and relation with the adjacent data is carried out by software. However, the burden on software is not significantly relaxed even in this method and there is a limit in enhancing the data writing speed.